The present invention relates to a two-component room temperature vulcanizable silicone rubber composition and, more specifically, relates to a novel cross-linking agent component for such two-component room temperature vulcanizable silicone rubber compositions.
Room temperature vulcanizable silicone rubber compositions are usually prepared and present either in a one-component form or two-component form. Generally, in the two-component room temperature vulcanizable silicon rubber compositions there is present a base silanol-stopped diorganopolysiloxane, filler and other additives, and stored as such. As part of the composition there is also formed a second component which must have the ingredients of a cross-linking agent such as a silicate, or partial hydrolysis products of such silicate, which is mixed with a catalyst which is usually a metal salt of a carboxylic acid. The second component is mixed and stored separately from the base composition which was defined above. When it is ready to cure the composition to form an elastomer, the cross-linking agent and catalyst mixture is mixed into the base composition and preferably thoroughly mixed into the base composition, and the resulting mixture is applied and allowed to cure at room temperature to produce a silicone elastomer.
The above description of a two-component room temperature vulcanizable silicone rubber composition does not describe all such types of compositions but, in a general way, describes the base composition for most two-part room temperature vulcanizable silicone rubber compositions available in the marketplace.
It must be emphasized that the catalyst and cross-linking component is kept separate from the base composition upon manufacture since if the catalyst cross-linking agent component is mixed with the base composition, then the material will start to cure immediately to form a silicone elastomer and will most likely be completely cured in 24 hours. Thus, for storage purposes, the cross-linking agent and catalyst component is maintained separately from the base component until it is desired to mix the components and apply them so as to produce a silicone elastomer.
The one-component room temperature vulcanizable silicone rubber compositions of which there are a great many variations, is formed by mixing the ingredients anhydrously and packaging this mixture as such in one package. When it is desired to cure the composition to form a silicone elastomer, the one-component system is applied and exposed to atmospheric moisture whereupon the one-component system will cure after due course into a silicone elastomer.
With this background it is necessary, with respect to the present invention, to look at the disclosure of the Lewis et al. patent, U.S. Pat. No. 3,186,963, where it is stated that with such two-component room temperature vulcanizable silicone rubber compositions, as described previously, there were three major difficulties which the inventors attempted to correct. It is disclosed that the catalyst and cross-linking component tends to have a high freezing point and, accordingly, it was difficult to use and mix this second component into the base component at low temperatures.
Another disadvantage, as alleged in Lewis et al., was that the catalyst and cross-linking agent component, when taken and mixed into the base component, that the concentration of ingredients would vary from mixture to mixture such that the cure rate would vary from the same batch of the base component.
The most serious disadvantage alleged in the Lewis et al. patent was that the catalyst component seriously degraded in activity upon storage.
Accordingly, Lewis et al. attempted to solve these problems in accordance with their invention by reacting the cross-linking agent, that is, the silicate, with the metal salt of a carboxylic acid at elevated temperatures and utilize the reaction product as the second component in a two-component room temperature vulcanizable silicone rubber composition. Although it was found that such a reaction product had good shelf stability for a limited period of time such as 1 or 2 months, it was still found that the activity of the catalyst reaction product would degrade undesirably over a period as long as 6 months or a year after manufacture. Also, prior to degradation of such a catalyst reaction product, it was found that at the early stages after manufacture of the reaction product that it was highly reactive such that small quantities of it, such as 0.5% to 1% by weight of the base component, was necessary to be utilized with the base component so as to get a desirably long work life of at least 5 minutes. Thus, it is desirable that the mixture have a work life of at least 5 minutes and preferably 10 minutes when mixed into the base composition and also with the use of larger amounts of the cross-linking agent component this is possible with the Lewis et al. reaction product.
As one solution proposed to solve this problem, it was suggested to utilize a dimethylpolysiloxane oil chain-stopped with trimethylsiloxy units or other triorganosiloxy units as a dilutant non-reactive material which could be added to the reaction product of Lewis et al., such that the resulting mixture could be used in larger amounts such as 2% to 30% by weight of the base component. This would allow the second component to be carefully metered into the base component composition to obtain a desirable work life and still utilize a large enough amount of the diluted cross-linking agent component so it could be utilized with facility in the preparation of room temperature vulcanizable silicone rubber compositions. The difficulty with utilizing such a dilutant was that the shelf life of the reaction product catalyst component degraded seriously in that the shelf life was less than the 1 or 2 months.
It is one object of the present invention to produce a catalyst component for a two-component room temperature vulcanizable silicone rubber composition which has standard shelf stability, such as 6 months to a year.
It is another object of the present invention to produce a catalyst component for a two-component room temperature vulcanizale silicone rubber composition in which the catalyst component can be utilized in sufficient large amounts so it can be easily mixed into the base component and has the desired reactivity.
It is an additional object of the present invention to provide an efficient and simple process for producing a catalyst component for a two-component room temperature vulcanizable silicone rubber composition wherein the catalyst component imparts to the resulting uncured mixture, that is, when the catalyst component is mixed with the base component, the desired work life.
It is still an additional object of the present invention to provide a catalyst component reaction product for a two-component room temperature vulcanizable silicone rubber composition, which reaction product is the same irrespective of the different samples that may be taken from the catalyst component reaction product mixture.
It is yet an additional object of the present invention to provide for a two-component room temperature vulcanizable silicone rubber composition which is uniform and has the properties of prolonged shelf life, desirable reactivity and uniformity in application.
These and other objects of the present invention are accomplished by means of the disclosure set forth herein below.